1. Field of the Invention
The present invention relates to a constant voltage control device that supplies difference types of stabilized voltages such as DC 5V and DC 3.3V or DC 2.5V from an on-vehicle battery to an on-vehicle electronic device including a microprocessor and various memories and, more particularly, to an improved constant voltage control device for decreasing power consumption of a constant voltage-controlling power transistor to suppress heat generation.
2. Description of the Related Arts
In order to achieve higher-speed operation and reduction in power consumption of a microprocessor to be mounted on an on-vehicle electronic control device and the like, it has been a technique widely adopted in practical use to decrease a voltage applied to an operation part or various memories, thereby applying thereto a stabilized voltage different from that of an input/output circuit section.
For example, the Japanese Patent Publication (unexamined) No. 352675/2001 “Power Supply for On-Vehicle Computing Unit” proposes a power supply circuit in which a first switching element for obtaining a stabilized voltage of DC 5V from an on-vehicle battery and a second switching element for obtaining DC 3.3V are connected in series.
Further, the Japanese Patent Publication (unexamined) No. 316482/2003 “Power Supply Circuit” discloses means in which a switching-type primary power supply regulator is disposed for obtaining a primary power supply voltage from an on-vehicle battery, and a plurality of dropper-type secondary power supply regulators are connected to a primary voltage output of which voltage is dropped and stabilized, thereby obtaining various stabilized voltages of DC 5V or DC 3.3V.
Furthermore, the Japanese Patent No. 3143284 “DC Stabilized Power Supply Unit” discloses means in which a switching-type primary chopper regulator and a dropper-type secondary series regulator are connected in series to obtain a stabilized voltage of lower ripple fluctuation and lower power consumption.
Additionally, the Japanese Patent Publication (unexamined) No. 98874/1998 “DC Stabilized Power Supply” discloses means in which a switching-type primary chopper regulator and a dropper-type secondary series regulator are connected in parallel to obtain a stabilized voltage of lower ripple fluctuation and lower power consumption.
According to the above-mentioned Japanese Patent Publication (unexamined) No. 352675/2001 “On-Vehicle Computing Unit Power Supply”, both first and second switching elements are dropper-type voltage regulators, so that a problem exists in too much power consumption of the first switching element at the time of a high power supply voltage of an on-vehicle battery and a large load current.
In contrast, according to the Japanese Patent Publication (unexamined) No. 316482/2003 “Power Supply Circuit”, it is certain that a power consumption of dropper-type secondary power supply regulators for obtaining DC 5V is enormously reduced due to the series connection of a switching-type primary power supply regulator. But, for example, when an output voltage from the primary power supply regulator is DC 7V, a voltage drop amount of the dropper-type secondary power supply regulator for obtaining DC 3.3V is 7V−3.3V=3.7V, so that a problem exists in that a power consumption is not sufficiently reduced.
In particular, a voltage drop amount of the secondary power supply regulator for obtaining DC 2.5V is 7V−2.5=4.5V, resulting in too much power consumption. Thus, when disposing a plurality of switching-type primary power supply regulators, a problem exists in larger-sized and more expensive device.
On the other hand, “DC Stabilized Power supply Unit” of the Japanese Patent No. 3143284 and “DC Stabilized Power Supply” of the Japanese Patent Publication (unexamined) No. 98874/1998 are for obtaining a single stabilized voltage, and do not propose means for obtaining plural types of stabilized voltages.
In the case of “DC Stabilized Power Supply” of the Japanese Patent Publication (unexamined) No. 98874/1998, being the invention relevant to the present invention, a problem exists in that output voltages from a pair of regulators connected in parallel are difficult to be in coincidence accurately, and that over-voltage ripple of a switching-type primary chopper regulator cannot be eliminated.